Mass spectrometer with tandem ion mobility analyzers

ABSTRACT

The invention proposes a mass spectrometer comprising two ion mobility analyzers in tandem arrangement, of which at least one is a trapped ion mobility spectrometer (TIMS), and an ion gate which is located between the two ion mobility analyzers, and use thereof wherein ions are selectively transferred between the two ion mobility analyzers by adjusting the transmission of the ion gate while ions are separated in time according to ion mobility in the first ion mobility analyzer.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to mass spectrometers with tandem ion mobilityanalyzers, in particular comprising built-in trapped ion mobilityspectrometry (TIMS) analyzers, and corresponding methods for separatingions according to their mobility for detailed substance analyses.

Description of the Related Art

U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008) presents a small ionmobility analyzer/spectrometer which has become known under the acronym“TIMS” analyzer/spectrometer (TIMS=trapped ion mobility spectrometry).The terms ion mobility analyzer and spectrometer are usedinterchangeably here. A TIMS analyzer comprises a gas flow that drivesions against a counter-acting electric field barrier such that the ionsare at first trapped along the axis of the TIMS analyzer. The ions areconfined in the radial direction by an electric RF field. Aftertransferring ions from an ion source to the electric field barrier, theheight of the electric field barrier or the gas velocity is adjustedsuch that ion species are released from the electric field barrier inthe sequence of their mobility.

Commonly, the length of the ion mobility separation unit of a TIMSanalyzer amounts to about five centimeters only. In a small tube with aninner diameter of about eight millimeters, a radial RF quadrupole fieldis generated to hold ions near to the axis. A gas flow inside a tubedrives ions entrained in the gas flow against a ramped counter-actingelectric DC field barrier where the ions are trapped and separatedaccording to their mobilities at locations on the field ramp at whichthe friction force of the moving gas equals the counter-acting force ofthe electric DC field on the ramp. After loading the TIMS with ions, theheight of the electric DC field barrier is decreased; this scan releasesthe ion species in the sequence of their mobility. Unlike many othertrials to build small ion mobility spectrometers, the small device by M.A. Park has already achieved, with reduced scan speeds, ion mobilityresolutions up to R_(mob)=400, which is extraordinarily high.

FIG. 1 outlines schematically a common TIMS analyzer and its operation.Entrained by a gas (7), ions (6) from an electrospray ion source (notshown) are introduced via capillary (8) into a first chamber of a vacuumsystem. A repeller plate (9) drives the ions (6) into an entrance funnel(10) of the mobility analyzer. Ion funnels (10, 12) usually are built asa stack of apertured diaphragms the openings of which taper to smallerdiameters thus forming an inner volume in the shape of a funnel. Twophases of an RF voltage are applied alternately to the diaphragms tobuild up a pseudopotential which keeps the ions away from the funnelwalls. The ions are driven to and through the narrow end of the firstfunnel (10) into the TIMS tube (11) by an axial gas flow (14) andoptionally by an additional DC potential gradient along the diaphragms.

The axial gas flow (14) through the TIMS tube (11) is laminar and shows,in radial direction, a substantially parabolic velocity distribution.Nitrogen may serve as a preferred gas. The vacuum conditions around theTIMS tube (11) are chosen such that the maximum gas velocity amounts toabout 100 to 150 meters per second, at a pressure of a few hectopascals.This velocity is only achieved near the axis. Further off axis, thevelocity is considerably smaller, as indicated by the arrows (14) inFIG. 1.

The first funnel (10) guides the ions into the TIMS tube (11) forming atunnel with internal RF quadrupole field in radial direction. The TIMStunnel (11) comprises a stack of thin electrodes with central holeswhich form a circular tube arranged around the z-axis of the device. Thethin electrodes are separated by insulating material closing the gapsbetween the electrodes around the tube. The electrodes of the TIMS tube(11) are segmented into quadrants (1, 2, 3, 4), to allow for thegeneration of a radially confining quadrupolar electric RF field inside.The quadrants (1, 2, 3, 4) of the tube electrodes are shown at the topof FIG. 1 with equipotential lines of the quadrupolar RF field insidethe tube at a given time. It should be mentioned here that the design ofa quadrupole tunnel does not necessarily consist of metal electrodesheets; there are a lot of different possibilities including stacked PCBboards or even a rolled PCB board with printed electrodes.

Inside the TIMS tunnel (11), the ions are blown by the gas flow (14)against an axial electric DC field barrier. In the center part of FIG.1, the profile of the axial electric DC field barrier is shown for threephases of a scan. Between z locations (20) and (23), the electric DCfield increases linearly, generated by a quadratically increasingelectric potential. Between z locations (23) and (24), the electric DCfield remains constant, forming a plateau of the electric DC fieldbarrier, generated by a linear increase of the electrical potential. Ina simple device, for instance, the complete field profile can begenerated by a single voltage, applied to the diaphragm electrode atlocation (24), and divided by precision resistors along the diaphragmelectrodes of the TIMS tube (11). The resistors between location (20)and (23) increase linearly, the resistors between (23) and (24) haveequal resistance. In more complex devices, non-linear field electricfield profiles may be generated, even adjustable DC field profiles, e.g.by digital-to-analog converters (DAC).

The operation of the TIMS analyzer starts with an “ion accumulationphase”, accumulating ions on the uppermost electric DC field ramp of thediagram. A voltage difference on the order of 300 volt produces theelectric DC field barrier. The ions are blown by the gas flow,symbolically indicated by the arrows (16), against the electric DC fieldbarrier and are stopped there because they cannot surmount the electricDC field barrier. It should be noted that the arrows (16) represent themaximum gas velocity of the parabolic gas velocity distribution (14)within the tube. The ions are accumulated on the rising edge of theelectric DC field between locations (20) and (23), where ions of lowmobility (mainly heavy ions with large collision cross section) gatherin the high field near the upper end of the field ramp, whereas ions ofhigh mobility gather in the low field near the foot of the ramp. Thesize of the dots represents the abundance of the ions of distinct ionmobility, indicating the strength of the space charge. In the subsequent“scan phase”, the supply voltage for the electric DC field barrier issteadily decreased, and ions of increasing mobility can escape towardsan ion detector, particularly to a mass analyzer operating as iondetector. In the bottom of the figure, the resulting ion current of thereleased ion species is shown. The measured total ion current curvei=f(t) presents directly an ion mobility spectrum from low ionmobilities to high ion mobilities.

Regarding the theoretical basis of TIMS, see the research article“Fundamentals of Trapped Ion Mobility Spectrometry”, K. Michelmann, J.A. Silveira, M. E. Ridgeway and M. A. Park, J. Am. Soc. Mass Spectrom.,(2015) 26: 14-24 (published online: 21 Oct. 2014).

Improvements of the scan modes for TIMS analyzers have been made byapplication of non-linear scans to achieve a linear mobility scale, aconstant resolution along the mobility scale, or a temporal zoom (M. A.Park et al., U.S. Pat. No. 8,766,176 B2). Furthermore, U.S. patentapplication Ser. No. 15/341,250 (M. A. Park and O. Raether) describes aspatial zoom.

The ion mobility resolution R_(mob) depends on the scan speed. FIG. 3presents a typical function of the ion mobility resolution versus scanduration. The lower the scan speed, the higher the resolution. Asalready mentioned, ion mobilities of R_(mob)=400 have been achieved withthe comparably small devices, using slow scans. Since the ions generatedin the ion source are lost during the scan phases, the duty cycle (orthe utilization rate of the ions) depends on the ratio of theaccumulation time t_(a) to the scan time t_(s).

A TIMS analyzer with parallel ion accumulation is described in U.S.patent application Ser. No. 14/614,456 (“Trapping Ion MobilitySpectrometer with Parallel Accumulation”, M. A. Park and M. Schubert);it improves the utilization of the ions from the ion source to nearly100%. TIMS with parallel accumulation does in fact collect and separateby ion mobility all ions of the ion source without any losses of ions,as long as space charge effects do not impair further collection ofions. TIMS with parallel ion accumulation further provides the uniquepossibility to prolong the ion accumulation duration to find moredetectable ion species, thereby even increasing the ion mobilityresolution by a corresponding prolongation of the scan time. The ionsare collected in an accumulator unit, preferably almost identical to thescanning unit, at a ramp of an electric DC field barrier such that theyget spatially separated by their ion mobility along the ramp. Therefore,the accumulated ions are less influenced by space charge than in othertypes of accumulator units. Of greatest importance, however, is theunique feature of a TIMS analyzer that a longer accumulation periodpermits to increase the mobility resolution by choosing correspondinglylonger mobility scan durations, e.g. 100 milliseconds scan duration withan ion mobility resolution of R_(mob)=75 instead of 20 milliseconds scanduration with R_(mob)=30. As a consequence of the higher number of ionscollected and the better ion mobility resolution, more ion species canbe detected and measured. Once an ion mobility scan is completed(optionally after twenty to some hundred milliseconds), the accumulatedions are transferred (in about a millisecond) from the accumulation unitto the scanning unit, and the next ion mobility scan can be started. Intotal, a skilled practitioner will appreciate that it will be possibleto achieve a measurement rate of 300 to 450 ion species per second. IfTIMS with parallel ion accumulation is installed in tandem massspectrometer (MS/MS instrument) an MS-MS instrument, 300 to 450characteristic fragment ion spectra per second may be measuredquantitatively.

The major challenge with TIMS (as with other trapping spectrometers) isspace charge. Some improvements for higher amounts of stored ions inselected regions of ion mobility, particularly for ions of low ionmobility, are given in U.S. Pat. No. 9,304,106 B1 (M. A. Park and O.Raether, “High Duty Cycle Trapping Ion Mobility Spectrometer”). Thehigher loading capacity is based on non-linear electric DC field ramps,with flatter field ramps for ion species of interest, in order todiminish the effect of space charge for these ion species. But forprecise ion mobility analyses of low abundant ion species in complexmixtures the influence of the space charge is still too high.

There is still a need for a method to analyze precisely the mobility oflow abundant ions in the nearby presence of high abundant ions with highamount of space charge.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a method for analyzing ions ina mass spectrometer that comprises an ion source, two ion mobilityanalyzers, of which at least one is a trapped ion mobility spectrometry(TIMS) analyzer, an ion gate that is located between the two ionmobility analyzers and a mass analyzer. The method comprises the stepsof: separating ions in time according to mobility in the first ionmobility analyzer; selecting ions of interest by adjusting thetransmission of the ion gate during the separation in the first ionmobility analyzer; transferring the selected ions of interest to thesecond ion mobility analyzer; and separating the transferred ionsaccording to mobility in the second ion mobility analyzer. The step of“selecting ions of interest” comprises that substantially all ions of anion species of interest or that a portion thereof is selected. Thesecond ion mobility analyzer is preferably a TIMS analyzer, morepreferably both ion mobility analyzers are TIMS analyzers.

The separated ions can be further analyzed downstream of the second ionmobility analyzer by acquiring mass spectra or acquiring fragment massspectra. The acquisition of the fragment mass spectrum preferablycomprises selecting precursor ions in a mass filter that is locatedbetween the second ion mobility analyzer and a fragmentation cell.

In one embodiment, the second ion mobility analyzer is a TIMS analyzerand the first separation and the selectively transfer are repeated. Thesecond ion mobility analyzer is preferably operated to accumulate therepeatedly transferred ions of interest prior to separating themaccording to mobility. More preferably, the repeatedly transferred ionsare accumulated in an additional ion trap which is located between theion gate and the TIMS analyzer and used for decoupling the accumulationand the separation in the TIMS analyzer.

In another embodiment, the second ion mobility analyzer is a TIMSanalyzer and the transmission of the ion gate is adjusted while the ionsare separated in the first ion mobility analyzer such that the totalspace charge of the ions transferred to the second ion mobility analyzerand/or the local space charge of the transferred ions in the trappingregion of the second ion mobility analyzer is below a predeterminedthreshold.

In another embodiment, the transmission of the ion gate is adjustedwhile the ions are separated in the first ion mobility analyzer suchthat the transmission for a highly abundant ion species of interest islower than the transmission of less abundant ion species of interest.The transmission of the ion gate can be adjusted by varying the durationof opening intervals or by adjusting an effective opening of the iongate. The latter can be accomplished by varying DC and/or RF voltagessupplied to electrodes of the ion gate while the ions are separated inthe first ion mobility analyzer, in particular while an ion species isreleased from an electric field barrier of a first TIMS analyzer.

Any adjustment of the transmission of the ion gate is preferably takeninto account in a post-processing of ion mobility spectra and/or massspectra for evaluating the abundance of ion species and in a graphicalrepresentation of ion signals.

In another embodiment, only ions of a single range of mobility aretransferred to the second ion mobility analyzer, wherein the singlerange of mobility is a reduced subset of the full range of mobility. Theion gate is opened and closed once while the ions are separated in thefirst ion mobility analyzer such that the transferred ions of interestare substantially from the single mobility range. In case the second ionmobility analyzer is a TIMS analyzer, the electric DC field ramp of theTIMS analyzer is preferably adjusted such that the transferred ions ofinterest of the single mobility range spread substantially over the fullwidth of the electric DC field ramp. In addition, the steps of releasingions from the first ion mobility analyzer and transferring ions to thesecond TIMS analyzer can be repeated several times, before therepeatedly accumulated ions are released from the second TIMS analyzerfor further analysis.

In another embodiment, ions are transferred from the ion source towardsthe first ion mobility analyzer and trapped upstream of the first ionmobility analyzer while preceding ions are separated in the first ionmobility analyzer, in particular in an TIMS analyzer.

In a second aspect, the invention provides a method for analyzing ionsin a mass spectrometer that comprise an ion source, two ion mobilityanalyzers, of which at least the second ion mobility analyzer is atrapped ion mobility spectrometry (TIMS) analyzer, a mass filter locatedbetween the two ion mobility analyzers and a mass analyzer. The methodcomprises the steps of: separating ions in time according to mobility inthe first ion mobility analyzer; filtering the separated ions accordingto mass in the mass filter wherein the transmitted mass range of themass filter varies while the ions are separated in the first ionmobility analyzer; transferring the filtered ions to the second ionmobility analyzer; and separating the transferred ions according tomobility in the second ion mobility analyzer.

The second ion mobility analyzer is preferably a TIMS analyzer, morepreferably both ion mobility analyzers are TIMS analyzers.

The mass filter is one of low-pass filter, high-pass filter and aband-pass filter and is preferably operated at a gas pressure above 10Pascal, most preferably at the operating pressure of one of or both ionmobility analyzers.

The separated ions can be further analyzed downstream of the second ionmobility analyzer by acquiring mass spectra or acquiring fragment massspectra. The acquisition of the fragment mass spectrum preferablycomprises a selection of precursor ions in an additional mass filterthat is located between the second ion mobility analyzer and afragmentation cell.

In a third aspect, the invention provides a mass spectrometer thatcomprises an ion source, two ion mobility analyzers, of which at leastone is a trapped ion mobility spectrometry analyzer (TIMS), an ion gateor a mass filter located between the two ion mobility analyzers and amass analyzer. The ion gate is configured to vary the transmission ofions during operation of the first mobility analyzer. The mass filter isconfigured to vary the transmitted mass range during operation of thefirst mobility analyzer. Preferably, the second ion mobility analyzer isa TIMS analyzer, more preferably both ion mobility analyzers are TIMSanalyzers.

The mass spectrometer according to the invention can further comprise anion trap between the ion gate or mass filter and the second ion mobilityanalyzer, a fragmentation cell between the second ion mobility analyzerand the mass analyzer and an additional mass filter between the secondion mobility analyzer and the fragmentation cell. The additional massfilter can be a quadrupole mass filter. The mass analyzer of the massspectrometer can be one of a time-of-flight mass analyzer, atime-of-flight mass analyzer with orthogonal ion injection, a RF iontrap, a DC ion trap (like an orbitrap or cassini-trap) andion-cyclotron-resonance trap. The fragmentation cell can use at leastone of following fragmentation techniques: collision induceddissociation (CID), surface induced dissociation (SID),photodissociation (PD), electron capture dissociation (ECD),electron-transfer dissociation, and electron impact dissociation (EID).

The ion gate is preferably one of an ion-optical einzel lens and aBradbury-Nielsen grid wherein a DC and/or RF generator is connected tothe grid. A Bradbury-Nielsen gate operated with RF voltages presents theadditional advantage to reflect ions of high mobility while transmittingions of low mobility. The mass filter is preferably a Loeb-Eiber filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the device and operation of a common trapped ion mobilityspectrometry (TIMS) analyzer. Top: Scheme of the TIMS device. Center:The profile of the electric field strength along the z axis. The size ofthe dots on the electric field ramp reflects their space charge. Thescan releases the ion bunches in sequence of their mobilities. Bottom:The ion current of ion pulses separated in time according to mobility,representing an ion mobility spectrum.

FIG. 2 shows a schematic overview of a mass spectrometer according tothe invention with two TIMS analyzers and an ion gate in between.

FIG. 3 presents the measured ion mobility resolution R_(mob) for thefirst TIMS analyzer shown in FIG. 2, given for ions of low mobility(K≈0.5 m²/Vs), as a function of the scan duration. The scan duration isdefined here as the full scan time from low mobilities (K≈0.5 m²/Vs) tohigh mobilities (K≈1.0 m²/Vs). The curve is typical for TIMS.

FIG. 4 schematically depicts a device and operation of the device of theinvention, with a first TIMS analyzer (TIMS 1), an ion gate, and asecond TIMS analyzer (TIMS 2). The diagram below shows ions collected onthe field ramp of TIMS1, with a marked range of ion mobilityrepresenting the ions of interest. The size of the dots represents theamount of ions of each type, thereby indicating the space charge ofthese ions. During the scan these ions are selected by the gate andcollected on the electric field ramp of TIMS 2. They can be analyzedwithout space charge disturbance.

FIG. 5 presents an operation method for transferring ions from twodisjoint ranges of ion mobility from the first TIMS analyzer to thesecond TIMS analyzer by opening the ion gate two times.

FIG. 6 presents an operation method for transferring ions of interestfrom the first TIMS analyzer to the second TIMS analyzer wherein theelectric DC field ramp of the second TIMS analyzer is matched to collectthe ions of interest spatially separated as far as possible. Repeatingcollection over several scan periods of the first TIMS analyzer canincrease the number of ions collected in the second TIMS analyzer.

FIG. 7 presents an arrangement with an additional ion trap between theion source and the first TIMS analyzer for parallel ion accumulation andscan.

FIG. 8 presents an operation method in which ions of high abundancewithin the range of interest are partially transferred from the firstTIMS analyzer to the second TIMS analyzer by closing the gate in asuitable time interval during the release of these ions.

FIG. 9 shows a Bradbury-Nielsen grid on a printed circuit board as iongate.

FIG. 10 shows the distribution of the RF pseudo potential for a sectionof the Bradbury-Nielsen grid with saddle-like passageways for ions ofsufficiently low mobility.

FIG. 11 outlines the release of ion species of different masses andmobilities during a single TIMS scan. With an RF Bradbury-Nielsen gridoperated with decreasing voltage (RF gate control voltage) during theTIMS scan, the singly charged ions may be completely suppressed.

DETAILED DESCRIPTION

The invention provides a method for measuring ion mobility and massesusing a mass spectrometer comprising two ion mobility analyzers in atandem arrangement, at least one of those being a trapped ion mobilityspectrometry (TIMS) analyzer, and the ion mobility analyzers beingseparated by a fast-switching ion gate. The first ion mobility analyzeris scanned and thereby releases ions in sequence of their ion mobility.During the scan, the ion gate is operated such that ions from ionmobility ranges without interest are stopped and ions from mobilityranges of interest are transferred to the second ion mobility analyzerfor further analysis. The transferred ions enter the second ion mobilityanalyzer where they can be analyzed according to their ion mobility. Themass spectrometer then can measure their precise masses; a tandem massspectrometer even can measure fragment ion spectra for betteridentification.

In one embodiment, ion pulses of highly abundant ions in the range ofinterest are, preferably by a suitable opening interval of the ion gateshorter than the ion pulse length, only partially transferred to thesecond ion mobility analyzer such that the passed ions can be analyzedaccording to their ion mobility with high ion mobility resolution,undisturbed by space charge. Ions outside the range of interest aresubstantially reflected in full. It is an advantage of this invention tolower drastically the space charge influence onto the scan of a secondTIMS analyzer, and to allow for high amounts of ions in preceding ionmobility analyzers. As a result, the final ion mobility scan has ahigher ion mobility resolution, even with much higher amounts of ions,and the mobility spectrum or mobility-mass spectrum is more stable withrespect to mobility peaks vs. scan time.

In another embodiment, ions of several ranges of mobility aretransferred, by suitable opening the ion gate, to the second ionmobility analyzer during a single scan of the first ion mobilityanalyzer.

In another embodiment, ions of a single range of mobility only aretransferred, by opening the ion gate, to the second ion mobilityanalyzer. If the second ion mobility analyzer is a TIMS analyzer, thenthe electric field ramp of this TIMS analyzer can be adjusted such thatthe ions of the selected single ion mobility range of interest spreadapproximately over the full width of the electric field ramp. Ions canbe accumulated and scanned several times in the first ion mobilityanalyzer, transferring each time ions of interest to the second ionmobility analyzer, before the ions are scanned from the second ionmobility analyzer for further analysis.

The first ion mobility analyzer can use the drift of an ion bunchthrough a resting gas, but in a preferred embodiment, two TIMS analyzersare arranged in sequence (TIMS1 and TIMS2). The temporal scan of theTIMS analyzers may be linear or curved, or may be performed withtemporal zoom as described in U.S. Pat. No. 8,766,176 B2 cited above.The ramps of the electric field barriers may be curved to lower thespace charge at the upper end of the ramp as described in U.S. Pat. No.9,304,106 B1 cited above.

FIG. 2 shows a time-of-flight mass spectrometer with two TIMS analyzersin a tandem arrangement. The first ion mobility analyzer (TIMS 1) mayscan a bunch of collected ions, thereby releasing ions in sequence oftheir ion mobilities. During the scan, the ion gate is alternatelyopened and closed, closed to reflect (or neutralize) unwanted ions andopened to pass ions in distinct ranges of mobility. In this way, highlyabundant ions with their space charge can be reflected in full or atleast partially. The passing ions enter the second ion mobility analyzer(TIMS 2) where they can be analyzed according to their ion mobility withhigh ion mobility resolution, undisturbed by space charge. The massspectrometer then can measure their precise masses; a tandem massspectrometer with quadrupole mass filter and time-of-flight analyzer, aspresented in FIG. 2, even can measure fragment ion spectra for a betteridentification of the ion species.

The ions transferred to TIMS 2 may stem from a single range of mobility,or from several ranges, selected by switching the gate accordingly. Inone embodiment, ions from a single range of mobility are collected on aflat ramp of TIMS 2, to spread the ions as far as possible along the zaxis of TIMS 2. In this mode, ions may be accumulated and scanned inTIMS 1 several times to accumulate as many ions in TIMS 2 as requiredfor an analysis of high quality. If there is a kind of ion withextremely high abundance within the range of interest, only a smallportion of these ions may be transferred by reflecting the largest partof these ions. The ion gate preferably switches faster than the temporalwidth of ion pulses leaving the first ion mobility analyzer. The lengthof an ion pulse released by the scan is in the order of a millisecond,whereas the switching time for the gate can be below a microsecond; theion pulse therefore can easily be cut into portions.

Whereas the radially confining RF field of TIMS 2 is preferablyquadrupolar in order to achieve a high ion mobility resolution, TIMS 1may show a tube with higher inner diameter, and/or with radial RF fieldsof higher multitude, like hexapole, octopole, or dodecapole, or with anRF tunnel. An ion trap can additionally be located upstream of TIMS 1 toaccumulate ions from the ion source during the scan of TIMS 1. If thetrap can be mass selectively unloaded, even TIMS 1 may be relieved fromspace charge.

FIG. 3 presents the measured dependence of the ion mobility resolutionfor TIMS 1, with ion gate and TIMS 2 open for ion transmission. Themeasured ion mobility resolution of TIMS 2 is slightly lower due to theslight pressure diminution along the two TIMS devices (R_(mob)=140instead of R_(mob)=145 for 1000 milliseconds scan duration).

FIG. 4 illustrates, in the top part, two tandem TIMS devices (TIMS 1,TIMS 2) separated by an ion gate. In this embodiment, the ion gate isformed as an ion-optical einzel lens. The gate can be switched on andoff in less than a microsecond. During the scan of TIMS 1, the gate maybe opened and closed in a suitable manner to pick out ions of interest.The ions of interest transferred to TIMS 2 may stem from a single rangeof mobility, as shown in the bottom part of FIG. 4.

FIG. 5 shows a preferred mode of operation with more than one ionmobility ranges of interest. Here, ions of interest from two disjointmobility ranges are transferred from TIMS 1 to TIMS 2 in a single scanof TIMS 1 by opening and closing the ion gate two times during thesingle scan of TIMS 1.

A special mode of operation is illustrated in FIG. 6. Ions from a singlerange of mobility are collected on a flat ramp of TIMS 2, to spread theions as far as possible along the z axis of TIMS 2. In this mode, ionsmay even be accumulated and scanned in TIMS 1 several times toaccumulate as many ions in TIMS 2 as required for an analysis of highquality. Applying a temporal zoom in TIMS 1 can drastically shorten thetotal scan time of TIMS 1, by a fast scan down to the region of the ionsof interest (almost a jump), then scanning slowly the ions of interest,and then jumping down to get rid of the residual high mobility ions.Such a temporal zoom may shorten the total scan duration to about 10milliseconds, nevertheless achieving a mobility resolution of Rmob>100in TIMS 1 for neatly cutting out the ions of interest. Accumulating theions of multiple scans (e.g. 10 scans) in TIMS 2, allows for an ionmobility analysis in TIMS 2 with high ion mobility resolution.

The tandem TIMS devices described up to now have to be loaded with ionsfrom the ion source in an extra time phase, reducing the duty cycle ofthe instrument. In a preferred embodiment, there may be located anadditional ion trap between ion source and TIMS 1 to accumulate ionsfrom the ion source during the scan of TIMS 1, similar to the TIMS withparallel accumulation, described in U.S. patent application Ser. No.14/614,456 cited above. FIG. 7 presents a scheme of this device with anadditional ion trap. The ion trap may be any device capable to storeions. A preferred ion trap is a linear RF quadrupole ion guide, operatedwith gas at a pressure of some hundred Pascal, with DC barriers at bothends.

Accumulating the ions of multiple scans of TIMS 1 in TIMS 2, leads tothe problem that TIMS 2 can't accept ions from TIMS 1 while it is itselfscanning. The method has the drawback of a low duty cycle, reduced to50%. A 100% duty cycle can be achieved for a tandem TIMS device by usinga first ion trap between the ion source and TIMS 1 and a second ion trapbetween the ion gate and TIMS 2. Multiple scans of TIMS 1 (preferablynon-linear scans with temporal zoom) can deliver ions of a selectedmobility range or selected mobility ranges. These ions are accumulatedin the second ion trap, and, when the TIMS 2 is ready, these ions aretransferred from the second ion trap to TIMS 2 and there mobilityseparated. The second ion trap may be any device capable to store ions;however, in a preferred arrangement, a linear RF quadrupole ion guideserves as the ion trap, operated with gas at a pressure of some hundredPascal, with DC barriers at both ends.

If there is an ion species with extremely high abundance within the ionmobility range of interest, only a small portion of ions of this ionspecies can be transferred by reflecting, deflecting or neutralizing thelargest part of these ions, as shown schematically in FIG. 8. Theduration of an ion pulse released by a scan is in the order of onemillisecond, whereas the switching time for the gate is below amicrosecond; the ion pulse therefore can easily been cut into portions,as indicated symbolically in the figure.

To keep production costs low, TIMS 1 and TIMS 2 can be made identical.However, this is not absolutely required. Whereas the field inside TIMS2 should be quadrupolar to achieve a high ion mobility resolution, TIMS1 may show a tube with higher inner diameter, and/or with radial RFfields of higher multitude, like hexapole, octopole, or dodecapolefields. Even stacked ring ion guides may be used. In this way, TIMS 1can accumulate many more ions, and the scan is less influenced by spacecharge. A TIMS device of this kind is described in U.S. patentapplication Ser. No. 15/172,237 (Th. Betz, M. A. Park and O. Raether,2016).

The ion gate may be a simple ion einzel lens with three apertures, asindicated in FIG. 4. Other embodiments may comprise a Bradbury-Nielsengate operated with DC voltages. A Bradbury-Nielsen gate with wires whichcan be alternately supplied with positive and negative voltages ispresented in FIG. 9.

In an alternate operation mode, the Bradbury-Nielsen gate is operatedwith RF voltages, forming reflecting pseudo-potentials around the wires,as illustrated in FIG. 10. This RF Bradbury-Nielsen gate has theadditional advantage to reflect ions of low mass (high mobility) whiletransmitting ions of high mass (low mobility), the limit of transmissiondepending on the RF voltage on the wires (gate control voltage). Thiseffect may be used in diverse applications. As an example, FIG. 11depicts schematically mass spectra which are acquired for ion pulsesseparated according to ion mobility during a scan of TIMS 1. The singlycharged ions, forming a band in the lower part of the mass-mobilityplot, are quite often not well suited for further analysis. Using an RFBradbury-Nielsen gate with decreasing RF gate control voltage during thescan of TIMS 1, as indicated by the fat dashed line, keeps the singlycharged ions away from TIMS 2. This concept may be generalized such thatany undesirable species as for instance chemical background in thisrange may be separated from desired ion species above the dashed line inFIG. 11.

The arrangement of the invention, as shown in FIGS. 2 and 7, can also beused to investigate the effect of ion manipulations. As an example, anion species of distinct ion mobility, selected by the ion gate, may beaccelerated by 10 to 60 volts between ion gate and TIMS 2. The numerouscollisions with the molecules of the gas flow may change the molecularconfiguration, e.g. by partially unfolding highly folded ions. Theresulting change of their collisional cross section may then be analyzedby TIMS 2. As another example, different drift gases may be used in TIMS1 and TIMS 2, as for instance N₂ in TIMS 1 and CO₂ in TIMS 2. Oradditional gases (“modifiers”) may be used in TIMS 2, like SF₆ or smallorganic compound gases.

1. A method for analyzing ions in a mass spectrometer comprising an ionsource, first and second trapped ion mobility spectrometry (TIMS)analyzers, a mass filter located between the first and second TIMSanalyzers and a mass analyzer located downstream of the second TIMSanalyzer, the method comprising the steps of: transferring ions from theion source to the first TIMS analyzer; driving the ions transferred tothe first TIMS analyzer by a first gas flow against a firstcounter-acting electric DC field barrier such that the ions are trappedand spatially separated according to their mobilities at differentpositions along a ramp of the first electric field barrier at which afriction force of the first gas flow equals the counter-acting force ofthe first electric DC field barrier; temporally separating ionsaccording to mobility in the first TIMS analyzer by adjusting a heightof the first electric DC field barrier or the velocity of the first gasflow; separating undesirable ion mobility species from ion mobilityspecies of interest by varying the transmitted mass range of the massfilter while the ions are separated in the first TIMS analyzer;transferring the filtered ions to the second TIMS analyzer; driving theions transferred to the second TIMS analyzer by a second gas flowagainst a second counter-acting electric DC field barrier such that theions are trapped and spatially separated according to their mobilitiesat different positions along a ramp of the second electric DC fieldbarrier at which a friction force of the second gas flow equals thecounter-acting force of the second electric DC field barrier; andseparating the transferred ions according to mobility in the second TIMSanalyzer by adjusting a height of the second electric DC field barrieror the velocity of the gas flow.
 2. The method according to claim 1,wherein the mass filter is operated at a gas pressure above 10 Pascal.3. The method according to claim 1, wherein the mass filter is operatedat a gas pressure of one of or both TIMS analyzers.
 4. The methodaccording to claim 1, wherein the undesirable ion mobility species aresingly charged.
 5. The method according to claim 1, wherein theundesirable ion mobility species are chemical background.
 6. The methodaccording to claim 1, wherein the ions which are separated in the secondTIMS analyzer are further analyzed by acquiring mass spectra or fragmentmass spectra.
 7. A mass spectrometer comprising an ion source, first andsecond trapped ion mobility spectrometry analyzers (TIMS), a mass filterlocated between the two ion mobility analyzers and a mass analyzer,wherein the first and second TIMS analyzers each comprise a gas flow anda counter-acting electric DC field barrier configured to spatiallyseparate ions according to their mobilities at different positions alonga ramp of the electric field barrier at which a friction force of thefirst gas flow equals the counter-acting force of the electric DC fieldbarrier and to temporally separate ions according to mobility byadjusting a height of the electric DC field barrier or a velocity of thegas flow, and wherein the mass filter is configured to vary thetransmitted mass range during operation of the first TIMS analyzer. 8.The mass spectrometer according to claim 7, wherein the mass filter isone of a low-pass filter, a high-pass filter and a band-pass filter. 9.The mass spectrometer according to claim 8, wherein the mass filter isoperated at a gas pressure above 10 Pascal.
 10. The mass spectrometeraccording to claim 8, wherein the mass filter is a Loeb-Eiber massfilter.
 11. The mass spectrometer according to claim 7, furthercomprising a fragmentation cell between the second TIMS analyzer and themass analyzer.